Make messages searchable

When a message is fully reassembled, extract a list of words from the message body and place them in a separate DB column, which can be full-text indexed.

Feedback from @Pratiwir: "We should also be able to search the chat as it might have a lot of text."

mentioned in issue #1199 (closed)

A user asked for the ability to search forums and blogs.

mentioned in issue #1642 (closed)

changed the description

added Help wanted S: stale UI/UX labels

added Usability label